In an electrostatic or electrophotographic printer of a document reproduction apparatus, documents are generally reproduced by using data derived from analysis of such a document to form an electrostatic image thereof on a support by means of an electrostatic or electrophotographic printing method depending on the type of printer, by developing said latent image by means of a development substance and fixing the developed latent image directly on said support or transferring the image onto another support (e.g. paper) and fixing thereon the image which has been developed and transferred.
In numerous printers of these types, the latent image is developed in what is currently called a developer unit by bringing coloured (e.g. black) particles of a powder developer substance close to the support of the latent image to be developed so that electrostatic forces due to the charges forming the latent image attract the developer particles towards the support and hold them on its surface at those places where the charge of the image is located, thereby making the latent image visible.
Developer methods currently used and known under the name of the "magnetic brush" method such as the method described in U.S. Pat. No. 2,786,439 use a powdered developer substance made of magnetic particles. The developer substance can be of the two component type in which case it is constituted by a mixture of fine coloured non-magnetic particles and larger magnetic particles-usually iron particles. Alternatively, said substance can be of the single component type in which case it is constituted by coloured particles which contain a magnetic compound such as, for example, magnetite. In accordance with a known method, described in U.S. Pat. No. 4,155,328, under the effect of magnetic drive forces, a particular configuration which is reminiscent of that of a rotary brush is imparted to the developer powder which is drawn e.g. from a tank. The support of the latent image moves past the "brush" thus formed which extends over the entire width of the support in a direction perpendicular to support movement whereby the entire surface of the support is swept by the "brush" which then leaves coloured particles of developer powder where the charges are located, said magnetically formed or magnetic "brush" being constantly regenerated to compensate for the loss of particles.
In accordance with said U.S. Pat. No. 4,155,328, a developer unit is known which uses such a method and a single component type development substance. In said developer unit the apparatus used to constitute the magnetic "brush" has a solid bar with a longitudinal axis of symmetry, said bar being assembled so as to rotate about said axis and bearing permanent magnets on its periphery. A cylindrical aluminium tube surrounds the bar fitted with the magnets, and a motor assembled in the developer unit in a stationary configuration rotates the bar by means of a coupling between the motor and the bar. The magnets each extend parallel to the axis along the entire bar except for the ends thereof which are thinner than the rest of the bar. These ends are mounted in two bearings and one of them is connected to the motor by the coupling means. The magnets are disposed uniformly around the periphery of the bar and are arranged so as to each have at its end nearest the tube a pole of a type different to that of the two magnets on either side of it.
A tank supplies the tube with powdered developer and under the effect of these magnets which rotate as one piece with the bar the developer substance forms a layer of particles around the tube which layer rotates in the opposite direction to the bar and constitutes said "brush" whose length is substantially the same as that of the magnets.
The present invention aims to allow a lighter, more compact and cheaper apparatus to be produced to form such a "brush".